Multi-collector mass spectrometers are well-known, particularly for distinguishing between isotopes. A mass-to-charge dispersive element, such as a magnetic sector, separates ions spatially according to their mass-to-charge ratios. Multi-collector measurements are the preferred method for high precision isotope ratio measurement, because the parallel detection of the isotopes or isotopologues may eliminate the effect of signal fluctuations on the precision of the measured isotope ratio. The detector array may be fixed or movable. An example of such a mass spectrometer is described in EP-0 952 607 A2.
The parallel measurement means that signal fluctuations are recorded on all isotopes or isotopologues at the same time. Thus, these signal fluctuations cancel for the isotope ratio calculation of two parallel measured signals. For large dynamic range isotope ratio measurements, this can present a particular challenge. The scattered background of the major ion beam (or beams) can disturb the baseline of the minor isotopes or isotopologues. This can be caused by scattering of the major ion beam, especially on apertures, slits, flight tubes in the magnetic field and along the optical flight path, residual gas molecules and on the detector modules. The scattered background typically is in the range of several parts-per-million (ppm) up to hundreds of ppm, depending on the relative mass distance of the major ion beams. This can make it difficult to perform high precision measurements (with a precision and accuracy in the range of 1% or smaller) of isotope ratios or isotope abundances with a dynamic range of 1:100 or greater.
In fact, existing instruments have been designed to measure isotope ratios with a maximum dynamic range of approximately 104. However, it is desirable to measure additional peaks at positions where two or more minor isotopes of one element are present in a molecule and thus create a very rare species, with abundances in the parts per million (ppm) range. In the case of carbon, oxygen and hydrogen, the heavier isotopes are the less abundant isotopes. If the molecule is formed at low temperatures, then there is a statistical preference to form molecules comprising of heavier isotopes. This effect is called clumping of isotopes in molecules. As these isotopes and particularly isotopologues are much less intense than others, a dynamic range between the collectors in the range of 106 to 108 is more preferable.
A particular example of clumped isotopes is in carbon dioxide testing with the measurement of the very rare 13C18O16O isotopologue alongside the major 12C16O16O isotopologue, which causes scattered background. The abundance of the 13C18O16O isotopologue is only in the range of a few ppm. The enrichment of this isotopologue can be used as an independent tracer for the formation temperature of this molecule. Hence, the precise and accurate abundance measurement of this isotopologue can be of fundamental interest for many fields of science, such as atmospheric sciences or applications in biogeochemistry. One way to eliminate the scattered background is to use an energy filter in front of the 13C18O16O detector to discriminate the scattered background, due to the small energy loss that the scattered ions have experienced by the scattering event. Examples of this are discussed in U.S. Pat. Nos. 5,180,913 and 5,043,575.
Another example where isotope ratios are measured over a large dynamic range is the measurement of uranium (U) isotopes. The accurate and precise measurement of the minor U isotopes is also challenging in view of their very low abundance. For instance, the isotope 236U only occurs naturally in very minor traces and a small enrichment of this tracer can therefore be regarded as an indicator as to whether the U material has been processed in a nuclear reactor. Consequently, the precise and accurate measurement of the 236U isotope can be important for nuclear forensics as well as for balancing of nuclear material through international control agencies. Isotopic ratio measurement of U may therefore demand a mass spectrometer with a high dynamic range.
Measurement of the minor U isotopes is made difficult by scattered background resulting from the major 238U isotopes. A known solution to this problem, which allows precise measurement of the minor U isotopes 234U and 236U, is to use a single or dual Retarding Potential Quadrupole (RPQ) setup. Here, the scattered background of the major 238U isotopes is discriminated by an energy filter, positioned in front of (upstream) the detector. Referring to FIG. 1, there is shown a diagram of an existing multiple collection detector configuration for the measurement of U isotopes. This comprises Compact Discrete Dynode (CDD) detectors, Faraday cups and Secondary Electron Multiplier (SEM) detectors. An arrangement is shown for detection of 233U, 234U, 235U, 236U and 238U isotopes, with an RPQ filter upstream the SEM detectors for the low abundance 234U and 236U isotopes. This procedure works, but requires a rather complex detector setup. Moreover, detection of U isotopes can present other issues, such as: ionization efficiency (sensitivity); fractionation corrections; detector uncertainties (noise, calibrations); peak tailing corrections; possible interferences (e.g. K-clusters); blank corrections; and availability of certified reference materials.
Referring to FIG. 2, there is shown an example mass spectrum from mass 233 to 238 for a sample measured on an existing thermal ionization mass spectrometer. The peak corresponding with 236U can be seen, with much lower abundance comprises with the 234U and 235U peaks. The problem of potassium (K) cluster interferences, specifically 39K, is shown. In the main spectrum shown, an RPQ setup has been used. In the corner, there is also shown a portion of the mass spectrum without the use of RPQ. The 236U peak is no longer distinct.
A further approach to overcome the background problem is to measure the background intensity slightly beside the peak, as discussed in “Long-term performance of the Kiel carbonate device with a new correction scheme for clumped isotope measurements”, A. Nele Meckler, Martin Ziegler, M. Isabel Milian, Sebastian F. M. Breitenbach and Stefano M. Bernasconi, Rapid Commun. Mass Spectrom. 2014, 28, 1705-1715. This can be achieved by setting the magnet of the sector to a slightly higher or lower mass, such that the detector does not catch the isotopologue, but instead measures the background beside the peaks. From this off-peak measurement, the background signal for the on-peak measurements can be deduced. Such a procedure can allow control of the background, but requires sequential background measurements during the sample measurement, which adds to the measurement time and to the uncertainty of the background measurement.
Moreover, it unnecessarily consumes sample material, which is very often limited. It has been found that the actual baseline may depend on the ion beam intensity and as such, the baseline would need to be measured and calibrated for different ion beam intensities. This is a very tedious procedure which limits the attainable precision and also further extends measurement time and reduces sample utilization. In reality, the user may not readily find the accurate baseline position, which is representative for the background measurement during the on-peak isotope ratio analysis. Finding the right baseline position can require extensive and elaborate calibration measurements and may be very time-consuming. Also, the baseline structure may change over time and also from instrument to instrument and can be hard to control.
Another way to eliminate or reduce the scattered background intensity is to shield the major ion beam in a type of split flight tube. This principle is implemented in a noble gas mass spectrometer manufactured by Thermo Fisher Scientific™ under the label Helix SFT™. This instrument is designed for the precise and accurate measurement of the 4He and 3He isotope abundances and uses a split flight tube starting at the exit of the magnet. The 4He and 3He ion beams are guided into separate flight tubes, such that the scattered background due to the major 4He ion beam is stopped in the 4He flight tube and does not cause significant interference to detection of the relatively very minor 3He beam. Like the energy filter setup, the split flight tube approach also requires rather complicated and complex detector configurations, which dramatically reduce the flexibility of the detector setup.
In view of the above, an improved approach for the accurate and precise measurement of isotope ratios with a high dynamic range is still being sought. Existing technologies are complex and therefore expensive to implement and may suffer from other disadvantages, such as reduced throughput and/or require higher sample quantity. The known approaches may also limit or restrict the flexibility of the instrument and the detector array for other applications.